Qualitative and Quantitative Changes of Carbonate Rocks Exposed to SC CO2 (Basque-Cantabrian Basin, Northern Spain)

This study aims at the qualitative and quantitative determination of porosity, mineralogical and textural changes in carbonate rock samples after injection of (i) supercritical CO2-rich brine and (ii) dry supercritical CO2, under similar experimental conditions (P ≈ 75 bar, T ≈ 35 °C, 970 h exposure time and no CO2 flow). The studied rocks were sampled in the western Basque-Cantabrian Basin, North Spain, and consist of vuggy carbonates (“Carniolas”) of the Puerto de la Palombera formation (Hettangian). Mineralogical and pore space characterization is completed using optical microscopy, scanning electron microscopy and optical image analysis. In addition, X-ray fluorescence analyses are performed to refine the mineralogical information and to obtain whole rock geochemical data and the brine composition is analysed before and after the experiment. Mineralogical and chemical results indicate that the carbonate rocks exposed to supercritical CO2 in dry conditions do not suffer significant changes. However, the injection of supercritical CO2-rich brine induces chemical and physical changes in the rock due to the high reactivity of calcite at the low pH conditions produced by the acidified brine. Numerical modelling validates the experimental observations. These results can be used to characterize the behaviour of carbonate rocks under conditions similar to the vicinity of a CO2 injection well. The results should be considered only at the scale of the studied samples and not at reservoir scale.